The concentration of ore such as iron ore into a product which can be economically shipped and processed is a difficult endeavor and has historically taken many forms and variations. Ideally, an efficient process will require the least possible capital, use the least possible energy, employ the least possible water, and generate minimal waste disposal problems. Energy is consumed in grinding and pumping, and capital requirements entail significant expenditures for grinders, screens, gravity or other separators, and waste treatment facilities. Large quantities of water are used in almost all parts of a beneficiation plant, and waste disposal problems are functions of both water usage and the composition and quantity of tailings generated.
The primary objective of an ore beneficiation plant is to increase the value content of the product from its natural state to a practical high value depending on the kind of ore and the physical location of the next processing or utilization plant. In the case of iron ore, specular hematite in particular as found at Mt. Wright, Quebec, an ore containing about 31.4% iron is upgraded to a concentrate of about 66.3% iron. Many different flow sheets have been proposed and used for various ores, but generally it has been extremely difficult to increase recovery efficiency beyond a certain point without economic sacrifice.
In a commonly used process, ore ground to a sepcified size is separated in spiral separators designed and adjusted to employ the statistical separation efects of gravity flow in water. Large quantities of water must be handled, and in the past have contributed to a disposal problem and frequently necessitated the undesirable loss of iron, along with the water, in the form of fine particles. The processing of large quantities of recycled water has necessitated the use of organic polymeric flocculants, which have in the past found their way back into the system to form nutrients for bacteria. The bacterial deposits can alter the flow patterns in the spiral separators.
Another common problem is the loss of particles known as locked middlings, i.e. particles containing significant quantities of iron (or other mineral value) because of the inability of the spiral separator to distinguish between portions of the gangue and the locked middlings. It has been though that locked middlings could not be reground in an autogenous mill; however, as will be explained below, the locked middlings not only can be reground, but significant improvements in efficiency are obtained by doing so. The typical process step for locked middlings prior to the present invention was to treat the locked middlings as similar to misplaced middlings, and recycle them to the beginning of the spiral series. In processes aiming for a product of high purity, such as, in the case of specular hematite, of over 66% iron, no locked middlings can be tolerated in the product; consequently, they have in the past been eventually discarded and the value in them lost.